1. Technical Field
The invention relates to an electronic circuit. Particularly, the invention relates to a direct current (DC) offset cancellation circuit.
2. Related Art
In an operational amplifier of a communication apparatus, when an input signal of a previous stage has a direct current (DC) offset error due to temperature or component mismatch, the error is also amplifier by a gain of the operational amplifier, and such phenomenon probably results in a fact that the operational amplifier enters a saturation mode and cannot achieve a best result. A commonly used method for cancelling the DC offset is to feed back a signal output from an output terminal of the operational amplifier to an input terminal of the operational amplifier, so as to achieve an effect of cancelling the DC offset. However, in the aforementioned method for cancelling the DC offset, since the feedback signal is directly added to the input signal of the operational amplifier, noise in the feedback circuit is also transmitted to the input terminal of the operational amplifier, and a noise figure at the input terminal of the operational amplifier is greatly increased, which increases an interference degree on signal. In order to cancel the DC offset and suppress the noise increase, some other methods have to be used, for example, to decrease an input resistance or increase an input transconductance of the operational amplifier. On the other hand, the above methods for reducing the noises may further increase a design area or power consumption of a chip.